Edge of leaf turn pale, help!!! (Setting + Plant picture included)

As the title say, this is the first time my amigos and I growing cannabis in this indoor-controlled environment.
We have did our research and so far these are the best settings we could come up with now.
Some of the setting might not be the best practice to get maximum harvest as we are having a budget and also experimenting on this, and just to be true, we will be more more more than happy already as long as we have grown anything.

And here we go.

First of all, this is the grow box, it's about 1000 x 400mm, temporary using this size as we will move it into a bigger environment when it grow.

Here is the setting that I have, do recommend if I need some others critical equipment.

1. Temperature & Humidity sensor
Generally we keep the humidity above 54% but it's dropping quite quick, maybe is my country weather.
(humid and hot for all years, no season, only season is raining season / no-raining season)
For temperature without light is about 27.8 degree Celsius while with light is 28.6 degree Celsius, the temperature is kinda constant, so day and night not much difference.

2. Mini fan inside the grow box

3. Soil Moisture & PH detector
For moisture, we do my best to keep it at the green area of the detector.
While for the pH, we water the plant with filtered tap water but adjust the pH with lime juice.

4. Exhaust fan for air flow in bottom & air flow out on top.

PROBLEM:
The strain we growing now is the Royal AK from Royal Queen Seed.

This is day 5, and you can clearly see the edge of the first two leave turn pale.

Temporary solution:
Seriously, we have no idea.
But after some googling, we suspect a nutrient insufficient, could be Mg or Fe, maybe? Pardon but we not really sure about these stuff.
Anyway this is what we do to help the plant.
A well-pH water, mix with the fertilizer below, and measure to around 400 ppm.

We are really trying hard here, and would really appreciate for any help we could get here.
Experience grower please, we need your help. T.T

Thank you everyone for make it until this point of the long post.
Appreciated.

What type of soil do you have it in? Most soils are preloaded with plenty of nutrients, sometimes even too much for young plants. It sounds like you are adding nutes besides and this is too much, and clearly as noted, your leaves are showing what looks like a burn already. You need a more accurate way to check the pH of your incoming fluids too, that cheap meter is not going to be accurate enough to adjust to 6.3 every time. That soil also seems to be quite wet and this could also be a problem, as this plant needs to dry out each time before watering, all the way to the bottom of the container.

I agree with Em.
First, lose that meter.
It is junk and unless you thoroughly clean those electrodes constantly, it is quite inaccurate.

The goal here is not to "keep it in the green" anyway.
What we want to do is give it enough water to sprout, then wait.
With that small a 'ling, in that large a pot, you will have to wait quite a while until the wee thing can transpire enough water to dry the soil.

To know when that is, watch the bottom fan leaves for any sign of wilt, a wee bit more flutter in the breeze is a good tell.
Then to see how much she needs, lift the pot.

Or, for a more accurate read. use a postal scale.
Weigh the pot filled with dryish soil and write that on the side. water it to runoff, and write that too. Now you know the range.
Do not water again until it's almost as light as the lesser weight.

The number one cause of infant mortality with new growers is suffocation.
When a plant is too small to drain the container it is in The water pools in the bottom and goes "stale". That is, it loses it's dissolved air and gets stagnant.
Roots need oxygen, so the wee 'ling yellows, then dies.

Next run, I suggest that you start the seed in a beer cup, and unless it's feminized, plant several.
When it needs watering in only 24 hours, it's ready to up-pot.
A small increase each time. And no worry, done right, there is no "transplant shock". Indeed there is a growth spurt when it finds fresh soil.

I'd also suggest an oscillating fan to replace that mini. A fixed breeze can cause windburn and allows some leaves to stay shadowed by others, With a variable direction air flow, all leaves get light, just like outdoors.
Don't recognize that light, but the lack of stretch says it'll do.

There is no deficiency here, in fact, in soil, deficiencies are quite rare.
As long as the PH is within range most soils have everything the plant needs.
So, DO NOT attempt to feed a small seedling.
In good soil, FFOF et al, I seldom add anything. but wait until the plants "ask".
If you feel you must "feed" her, know that force feeding does not work. Best to see that the soil is adequate, then wait until the second, or third set of true leaves sprout, and start sparingly with perhaps 1/4 strength. Feed is in quotes because that is not what we are actually doing when we fertilize.

I'll go away now, you are in good hands with Emilya.
Suggest that you look up her posts on proper watering.
And for most of your newbie kine question you will be well served by clicking on the first link in my signature and scrolling through the archives here.